The goal of this project has been to characterize the host immunological responses to infection with G. lamblia. We initially examined giardiasis patients for the presence of circulating anti-G. lamilia antibodies by an enzyme-linked immunosorbent assay. IgG anti-G. lamblia antibodies were detected in serum. We will examine body secretions for IgA anti-G. lamblia antibodies. The immunoreactive antigens to which these antibodies are directed were characterized. To examine the cellular immune responses in giardiasis patients, we are studying in vitro lymphocyte transformation in response to G. lamblia trophozoite antigen. The effector mechanism of cellular interaction with G. lamblia has been investigated with spontaneous cell-mediated and antibody-dependent cytotoxicity assays. Characterizing the humoral and cellular immunological responses in patients with giardiasis may elucidate how host defense mechanisms modulate giardiasis.